A sintered R-T-B based magnet, including an R2T14B type compound as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances.
As a sintered R-T-B based magnet loses its coercivity HcJ (which will be simply referred to herein as “HcJ”) at high temperatures, such a magnet will cause an irreversible flux loss. For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss.
It has been known that if RP in the R2T14B type compound phase of a sintered R-T-B based magnet is replaced with a heavy rare-earth element RH (Dy, Tb), the coercivity will increase. To achieve high coercivity even at a high temperature, it is effective to add a lot of such a heavy rare-earth element RH to the sintered R-T-B based magnet. However, if the light rare-earth element RL (Nd, Pr) of the sintered R-T-B based magnet is replaced with the heavy rare-earth element RH, the coercivity will certainly increase but the remanence Br (which will be simply referred to herein as “Br”) will decrease instead. Furthermore, as the heavy rare-earth element RH is one of rare natural, resources, its use should be cut down.
For these reasons, various methods for increasing the coercivity of a sintered R-T-B based magnet effectively with the addition of as small an amount of the heavy rare-earth element RH as possible have recently been researched and developed in order to avoid decreasing Br. The applicant of the present application already disclosed, in Patent Document No. 1, a method for diffusing a heavy rare-earth element RH such as Dy from the surface of a sintered R-T-B based magnet body deep inside the magnet while supplying the heavy rare-earth element RH onto the surface of the sintered R-T-B based magnet body (which will be referred to herein as an “evaporation diffusion process”).
The applicant of the present application also proposed, in Patent Document No. 2, a method for diffusing RH from an RH diffusion source, which is either foil or powder that contains RH, into a sintered R-T-B based magnet body by carrying out a heat treatment with the foil or powder brought in contact with the surface of the sintered R-T-B based magnet body. According to the method disclosed in Patent Document No. 2, if the RH diffusion source is foil, then the foil has a thickness of 1 to 50 μm. On the other hand, if the RH diffusion source is powder, a powder layer with a thickness of 1 to 50 μm is formed on the surface of the magnet using powder with a particle size of 1 to 50 μm. In this manner, a small amount of RH can be used efficiently and can be diffused inside of the sintered R-T-B based magnet body. In one example, pure Dy was used as the RH diffusion source.
Meanwhile, Patent Document No. 3 discloses a method in which a fine powder of an RH—Fe compound with a mean particle size of 100 nm to 50 μm is used as an RH diffusion source and dispersed in a solvent to obtain slurry and in which a heat treatment is carried out with the slurry applied onto the surface of a sintered R-T-B based magnet body. According to the method of Patent Document No. 3, by using a ferrous compound as the RH diffusion source, HcJ can be increased significantly. In addition, since the melting point decreases around the eutectic point, the heat treatment temperature can be lowered and the magnet is less affected by a variation in temperature during the heat treatment process. On top of that, by using slurry in which a fine powder of an RH compound with a mean particle size of 100 nm to 50 μm is dispersed in a solvent, an RH compound can be deposited uniformly onto the sintered R-T-B based magnet body. As a result, the RH can be diffused more uniformly through the heat treatment.
Patent Document No. 4 discloses a method for carrying out a heat treatment with a powder of an RH diffusion source, which is an alloy of a rare-earth element and a non-rare-earth element, put on the surface of a sintered R-T-B based magnet body. The powder includes, as its essential elements, a rare-earth element, Fe, Co and various other M elements. In Patent Document No. 4, the powder of the RH diffusion source is also dispersed in either an organic solvent or water and the slurry is also applied onto the surface of the sintered R-T-B based magnet body. According to Patent Document No. 4, the smaller the mean particle size of the powder, the higher the diffusion efficiency should be.
Patent Document No. 5 discloses a method for carrying out a heat treatment with an alloy powder including RH with a particle size of 10 μm or less and an iron group transition element used as an RH diffusion source and with the alloy powder applied onto the surface of a sintered R-T-B based magnet body by barrel painting method, for example.
Patent Document No. 6 says that if an RH oxide layer is formed on the inner surface of a heat treatment vessel and if a heat treatment is carried out with a sintered R-T-B based magnet body arranged in such a heat treatment vessel, then the inner surface of the heat treatment vessel and the sintered magnet body will not adhere or stick to each other even when they are in contact with each other, and Hcj can be increased because RH in the RH oxide layer is reduced and diffuses and enters the sintered magnet.